This invention relates to electrostatic printing and more particularly to an improved ink or liquid developer composition for converting latent electrostatic images into visible images.
In electrostatic printing, latent electrostatic images are formed on a photoconductive surface of a recording element by uniformly charging the surface thereof, as by a corona discharge device, followed by exposure to light in the desired image pattern. Such images may be developed by solid (powder) developers or by liquid developers.
Liquid developer compositions for use with electrostatic images generally comprise a dispersion of pigment or toner particles in a volatile, insulating liquid of high volume resistivity in excess of 10.sup.9 ohm-centimeters. Suitable insulating liquids include: aromatic hydrocarbons, such as benzene, toluene, and xylene; aliphatic hydrocarbons, such as hexane, cyclohexane and heptane; freons and halogenated hydrocarbons and silicone oils. The liquid developer is applied to the imaged surface, and the suspended toner particles become electrostatically charged and develop the latent image by migration under influence of the image charge. This is known as electrophoretic development.
In another type of electrostatic image development disclosed by Gundlach in U.S. Pat. No. 3,084,043, liquid developers having relatively low viscosity, low volatility, contrast in color in the usual case with the surface on which it will remain, and relatively high electrical conductivity (relatively low volume resistivity), are disclosed for converting the electrostatic latent image to a visible image. According to this method, liquid developer from a reservoir is deposited on a gravure roll and fills the depressions in the roll surface. Excess developer is removed from the lands between depressions, and as a receiving surface charged in image configuration passes against the gravure roll, developer is attracted from the depressions in image configuration by the charge. This method of development is referred to as polar liquid development.
In the above described electrostatic copying devices which use liquid inks or developers wherein the developer or ink is attracted from the depressions in a gravure roll or other type of device to the surface retaining the charged image or non-image areas, there is a minimum voltage from which the ink transfers from the ink applicator (gravure roll) to the image retention surface. The minimum voltage at which ink transfers from the ink applicator to the image retention surface, such as a photoreceptor, is called the threshold voltage. In the electrostatic copying processes which use liquid inks, the electrical conductivity of the ink determines the threshold voltage as well as the degree of contrast. Inks of high electrical conductivity usually exhibit high contrast development and require low threshold voltage. Accordingly, it is advantageous to increase the conductivity of the ink compositions to lower the threshold voltage and to improve the degree of contrast. Although prior conductive ink and liquid developer compositions are known, improved results in copying processes can be obtained by increasing the conductivity of the compositions and by providing improved ink compositions.